In general, axial piston pumps have long been known in the art. In one of its simplest forms, an axial piston pump might include a plurality of pistons that reciprocate parallel to the axis of rotation of a cylinder block. The cylinder block of the pump is turned by a drive shaft. The pistons are fitted to bores in the cylinder block, and are connected to piston shoes and a shoe plate, so that the shoes bear against an angled swash plate. As the cylinder block turns, the piston shoes follow the angled swash plate, causing the pistons to reciprocate with each revolution of the cylinder block. Inlet and outlet ports are arranged in a valve plate so that the pistons pass the inlet as they are pulled out and pass the outlet as they are forced back in. The displacement of axial piston pumps is determined by the size and number of pistons, as well as the stroke length, which is determined by the angle of the swash plate. length, which is determined by the angle of the swash plate.
In most applications, the drive shaft for the axial piston pump is connected directly to a power shaft, such as a crank shaft for an engine, so that the drive shaft turns at the same rate as the power shaft. Since in most applications the drive shaft rotation rate is not independently controlled, engineers have sought other ways to control the flow rate through such a pump by varying piston displacement. Variable displacement axial piston pumps are used to pump liquids, such as engine lubricating oil, and typically utilize a moveable yoke that is capable of changing the swash plate angle to increase or decrease piston stroke. The moveable yoke can be positioned by any of several relatively complex means, including manual control, servo control, pressure compensator control, and load sensing and pressure limitor control. The addition of a variable angle swash plate along with the moveable yoke and the mechanisms to drive the same results in an axial piston pump with the ability to control flow rate, but, unfortunately the added complexity and parts renders the variable displacement axial piston pump more vulnerable to failure and requires significantly more maintenance than relatively simple fixed displacement axial piston pumps.
The present invention is directed to providing a relatively simple and robust axial piston pump with the same ability to control flow rate as the relatively problematic variable displacement axial piston pumps of the prior art.